1. Field of the Invention
The invention relates to a method for generating a low-resolution gray-scale pattern representing a high-resolution original character image.
2. Description of the Related Art
Bilevel representation, or on/off representation, has been used in cathode ray tube display devices, plasma display devices, liquid crystal display devices and various printers to display or print dot matrix patterns of characters. In the bilevel representation, a binary value is assigned to each picture element (pel). Thus, each pel in the matrix represents black (i.e., foreground) or white (i.e., background). However, the bilevel representation in the matrix produces a stair-step appearance along non-vertical and non-horizontal lines. As the resolution of a displayed or printed image decreases, the stepped edges become larger and increasingly displeasing to the viewer.
Display systems utilizing a plurality of gray-scale levels have been developed to provide a more natural display of character. An article "The Display of Characters Using Gray Level Sampling Arrays", John E. Warnock, Communications of the ACM, Vol. 14, No. 3, 1980, pp. 302-307, and U.S. Pat. No. 4,158,200, Charles L. Seitz et al., Burroughs Corporation, disclose the above system utilizing a plurality of different gray-scale levels or levels of luminance.
FIG. 7 shows the concept of a display utilizing different gray-scale levels. A character pattern of a high resolution, such as 88.times.88 dots/character box, is stored in a font memory. It is assumed that the character pattern is displayed on a display device of a resolution of 11.times.11 dots/character. In this case, a sampling pattern 21 having 8.times.8 sampling windows is used. To convert the original character pattern of 88.times.88 dots to the character image of 8.times.8 dots, the number of black pels of a portion of the original character image surrounded by one sampling window is counted, and one of eight levels of gray-scale is assigned in accordance with the number of black pels within the sampling window, so that a gray scale pattern 29 is generated. It is used to control the levels of luminance of the display device.
Although a display utilizing gray-scale levels solves the stepped edge problem, it raises a new problem when a relatively complicated character, such as a kanji character including many horizontal and vertical lines, is displayed. Referring to FIG. 14, a kanji character 51 of high-resolution stored in a front memory is shown. In this case a sampling pattern having 16.times.16 sampling windows is used. In the manner shown in FIG. 7, a gray scale value is assigned to the number of black pels counted in each sampling window to generate a gray scale pattern 52 representing the original kanji character 51. And, the gray scale pattern 52 is supplied to the display device. It is apparent that a displayed pattern using the gray-scale pattern 52 indicates poor quality, includes horizontal lines contacting each other of the same gray levels and indicates inferior readability.